1. Field of the Invention
The present invention relates to a sheet conveyor for sheet conveyance. More particularly, the invention is directed to reducing noise produced in conjunction with clearance of loop formed in a sheet on a sheet conveyance path.
2. Description of the Related Arts
Copiers and printers, for example, are equipped with a paper feeding station for feeding sheets stacked on a paper feed cassette into an apparatus. The paper feeding station includes a pickup roller for picking up a top sheet of those stacked on the paper feed cassette, a paper feed roller for introducing the picked sheet from the paper feed cassette into the sheet conveyance path, and a separation roller disposed in opposed relation to the paper feed roller and preventing two or more overlapped sheets from being transported.
The separation roller is provided with a torque limiter on a rotary shaft thereof. When a rotational force exceeding a torque limit of the torque limiter is applied to the separation roller, the separation roller is brought into rotation.
It is noted here that the torque limit of the torque limiter is defined to be greater than a sheet-to-sheet friction force and to be smaller than a friction force between the sheet and the separation roller. That is, the torque limiter is adapted to prevent two or more sheets from being transported by the paper feed roller.
On the sheet conveyance path, a conveyance roller pair is disposed downstream of a paper feed/separation roller pair in a sheet conveyance direction. The paper feed/separation roller pair consists of the paper feed roller and the separation roller. The conveyance roller pair is rotated at a lower peripheral velocity than that of the paper feed/separation roller pair such that the sheet being transported is prevented from being stretched between these roller pairs. With such a difference in velocity, the paper feed/separation roller pair and the conveyance roller pair allow the formation of the loop in the sheet therebetween.
However, it is difficult to spare a substantial space for the loop formation on the sheet conveyance path. In addition, there is a ceiling to the increase of the loop size. Therefore, an arrangement is made such that the driven rotation of the paper feed roller is suspended when the loop reaches a given size.
FIG. 1 is a graph showing the change in peripheral velocities of the paper feed roller and the conveyance roller pair and the change in amount of loop formed in the sheet. This graph shows the change in the peripheral velocity A1 of the paper feed roller, the change in the peripheral velocity B1 of the conveyance roller pair and the change in the amount C1 of sheet loop.
The paper feed roller and the conveyance roller pair are actuated simultaneously. When a predetermined amount of loop is formed in the sheet, or after the lapse of a predetermined length of time from the entry of a leading end of the sheet into the conveyance roller pair, control is provided to suspend the rotation of the paper feed roller. After this suspension of rotation, however, the conveyance roller pair continues to rotate and hence, the sheet is quickly decreased in the amount of loop.
It is noted here that the torque limiter of the separation roller provides resistance against the sheet conveyance and hence, the sheet is stretched taut when cleared of the loop, thus producing a loud noise. It is known that the noise increases with the increase in velocity of clearing the loop (velocity at which the loop in the sheet is decreased in a direction perpendicular to a sheet surface).
According to an image forming apparatus disclosed in a patent document 1 (Japanese Unexamined Patent Publication No. 2005-154119), the following method is adopted to prevent the sheet from being slammed against a guide. In secondary sheet conveyance following the formation of loop in the sheet, a paper stop roller pair is driven in advance of the conveyance roller pair and drive units for driving the paper stop roller pair and the conveyance roller pair are so controlled as to vary stepwise the driving speeds for these roller pairs.
The image forming apparatus of the above patent document 1, however, is designed to increase the rotational speed of the paper stop roller pair stepwise and hence, the apparatus suffers a low startup speed of sheet conveyance by the paper stop roller pair, resulting in decrease in image formation efficiency. Therefore, the same problems such as the decrease in image formation efficiency may also occur in a case where the technique of the patent document 1 is applied to the sheet feeding station including the separation roller equipped with the torque limiter.